The description of this invention and its background will be approached in the context of well logging because well logging is a known application of the invention. There is no intention to limit the generality of the present invention to the field of well logging.
Fluid flow properties of porous media have long been of interest in the oil industry. A. Timur, in "Pulsed Nuclear Magnetic Resonance Studies of Porosity, Movable Fluid, and Permeability of Sandstones," Journal of Petroleum Technology, June 1969, p.775, proved experimentally that NMR methods provide a rapid nondestructive determination of porosity, movable fluid, and permeability of rock formation.
It is known that when an assembly of magnetic moments such as those of hydrogen nuclei are exposed to a static magnetic field they tend to align along the direction of the magnetic field, resulting in bulk magnetization. The rate at which equilibrium is established in such bulk magnetization upon provision of the state magnetic field is characterized by the parameter T 1, the spin-lattice relaxation time.
It has been observed that the mechanism which determines the value of T 1 depends on molecular cynamics. In liquids, molecular dynamics is a function of molecular size and intermolecular interactions. Therefore, water and different types of oil have different T 1 values.
In a heterogeneous medium such as porous solid which contains liquid in its pores, the dynamics of the molecules close to the solid surface are also significant and differ from the dynamics of the bulk liquid. It may thus be appreciated that the T 1 parameter provides valuable information relating to well logging parameters.
There exist a number of techniques for disturbing the equilibrium of an assembly of magnetic moments such as those of hydrogen nuclei, for T 1 parameter measurements. One such technique is exemplified by the Schlumberger Nuclear Magnetic Logging Tool.
The Schlumberger Nuclear Magnetic Logging (NML) tool is described in R. C. Herrick, S. H. Courturle, and D. L. Best, "An Improved Nuclear Magnetic Logging System and its Application to Formation Evaluation," PE 8361 presented at the 54th Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers of AlME, held in Las Vegas Nevada, Sept. 23-26, 1979 and in R. J. S. Brown et al. U.S. Pat. No. 3,213,357, entitled, "Earth formation and fluid material investigation by nuclear magnetism relaxation rate determination".
The Schlumberger Nuclear Magnetic Logging (NML) tool measures the free precession of proton nuclear magnetic moments in the earth's magnetic field by applying a relatively strong DC polarizing field to the surrounding rock formation in order to align proton spins approximately perpendicular to the earth's magnetic field. The polarizing field must be applied for a period roughly five times T1 (the spin-lattice relaxation time) for sufficient polarization (approximately 2 seconds). (See the Herrick et al. reference mentioned above.) At the end of polarization, the field is turned off rapidly. Since the proton spins are unable to follow this sudden change, they are left aligned perpendicular to the earth's magnetic field and precess about this field at the Larmor frequency corresponding to the local earth's magnetic field (roughly from 1300 to 2600 Hz, depending on location).
The spin precession induces, in a pickup coil, a sinusoidal signal whose amplitude is proportional to the density of protons present in the formation. The signal decays with a time constant T2* (transverse relaxation time) due to inhomogeneities in the local magnetic field over the sensing volume.
Hydrogen protons in solids or bound to surfaces have very short characteristic relaxation times T 1, however bulk fluids in pore spaces have much longer relaxation times. In view of the fact that the observed decay with a relaxation time constant T 2* is less than or equal to T 1, the Schlumberger NML tool is blind to matrix and bound protons by delaying observation of the signal until 20-30 milliseconds after the beginning of decay. T 1 measurements can be performed by comparison of free precession following polarizing pulses of differing duration. Because the large polarizing field cannot be turned off instantly, much of the signal amplitude is lost.
At present there are two ways to compensate for this effect:
1. U.S. Pat. No. 3,483,465, to J. M. Baker, entitled "Nuclear magnetic logging system utilizing an Oscillated Polarizing Field," employs a polarizing field which is allowed to oscillate at the Larmor frequency corresponding to the earth's magnetic field for a few cycles.
2. U.S. Pat. No. 3,667,035, to Slichter, entitled "Nuclear Magnetism Logging," describes applying an alternating magnetic field in a direction transverse to the earth's magnetic field and at a frequency corresponding to the Larmor precession frequency corresponding to the earth's magnetic field.
Although there have been major improvements in the Schlumberger nuclear magnetic logging (NML) technique during the last 25 years, the following disadvantages have not yet been overcome:
1. Species with short relaxation time (shorter than 20-30 msec) cannot be detected by the Schlumberger NML technique due to long dead time of the system following the polarizing DC pulse.
2. The Schlumberger NML technique involves the suppression of a very high undesired signal coming from the bore fluid (which is in close proximity to the probe) and requires doping of the bore fluid with paramagnetic materials. This process is costly and time consuming.
3. The Schlumberger NML technique cannot carry out a T 1 (spin-lattice relaxation time) measurement at a commercially operational logging speed due to the long time required for each single T 1 measurement.
Another technique for nondestructive determination of porosity, movable fluid, and permeability of rock formation is the Los Alamos NMR technique described in the following publications:
R. K. Cooper and J. A. Jackson "Remote (Inside-Out) NMR.I Production of a Region of Homogenous Magnetic Field," J. Magn. Reson. 41, 400 (1980); PA1 L. J. Burnett and J. A. Jackson, "Remote (Inside-Out) NMR. II Sensitivity of NMR Detection for External Samples," J. Magn. Reson. 41, 406(1980); PA1 J. A. Jackson, L. J. Burnett and J. F. Harmon, "Remote (Inside-Out) NMR. III Detection of Nuclear Magnetic Resonance in a Remotely Produced Region of Homogeneous Magnetic Field," J. Magn. Reson. 41, 411 (1980);
U.S. Pat. No. 4,350,955, to J. A. Jackson et al., entitled "Magnetic Resonance Apparatus."
The Los Alamos NMR technique is based on the development of a new type magnet/RF coil assembly. This allows one to obtain the NMR signal mostly from a torioidal "doughnut"-shaped region in the surrounding rock formation at a specified distance from the bore hole axis.
The Los Alamos approach is based on T 1 measurements only, which are achieved by standard pulse NMR techniques which allow one to overcome one of the difficulties noted above in connection with the Schlumberger technique, i.e. the problem of the long dead time. However, it does not eliminate the bore fluid signal problem nor does it overcome the difficulty of unacceptably low operational speeds due to low signal to noise ratio. Jackson proposes to increase significantly the static magnetic field strength but admits that this is impractical at the present state of magnet technology.
A basic difficulty with the Los Alamos approach of Jackson lies in the fact that there is defined a "doughnut"-shaped region having high homogeneity whose location and field strength tend to vary over time during operation due in part to changes in the local earth's magnetic field, temperature, and mechanical parameters of the field producing apparatus. The Los Alamos approach, employing an antenna which is tuned to a fixed frequency, lacks the flexibility to resonantly match the changing field.
A similar difficulty is encountered using a technique described in U.K. Patent Application No. 2,141,236A, published Dec. 12, 1984, entitled "Nuclear Magnetic Logging".
Another basic difficulty associated with any technique wherein a relatively small size, high homogeneity region is examined lies in the fact that each single measurement of the decay process must have a duration approximately as long as the relevant relaxation time. The tool must be present in the local region of measurement throughout this duration, thus restricting the logging operation to non-economical logging speeds.